confusion about user paring with pg_hba and pg_ident

I am a little confused about some of my settings when it comes to map
linux/psql users.

I have two databases: mattermost and thetradinghall
I have two linux users: mattermost and dovecot. mattermost is the one who
want to talk to mattermost and dovecot the one who want to talk to
thetradinghall.
I have two postresql users: mmuser and mailman.

Here are the relevant part of my config files:

pg_hba.conf
-------------------
local thetradinghall mailman peer
map=mailmap
local mattermost mmuser peer
map=mattermap

pg_ident.conf
----------------------
mailmap dovecot mailman
mattermap mattermost mmuser

* question: can I use same mapname for my both DB, or using two mapnames
like I did is the correct way?

Now testing:
-----------------------------
bash-4.3$ whoami
mattermost
bash-4.3$ psql postgres:///mattermost?
psql: FATAL: no pg_hba.conf entry for host "[local]", user "mattermost",
database "mattermost", SSL off
-----------------------------------

I thought my settings told postgres that linux user mattermost was mapped
by psql user mmuser (which of course holds the mattermost DB), but it seems
it is not the case.
What do I do wrong?

Thank you for help.